Cellulose esters of organic acids are useful for a variety of purposes. The most common among the cellulose esters is cellulose acetate, which is used extensively in textile and polymer industries, among others. Specific applications include cigarette tow, cellulose films, and packaging, particularly food packaging.
Cellulose esters are advantageous due in part to the renewable nature of the cellulose, the starting material for all cellulose esters. Although cellulose acetate has been produced from both cotton and wood pulp, wood pulp is the dominant cellulose source due to economic factors.
Typically, cellulose ester in general, and cellulose acetate in particular, is produced from high purity pulp—that is, cellulose pulp having an alpha cellulose content greater than about 95%. The demand for high purity pulp stems from the desired properties of the products from which cellulose esters are made. These desired properties include, for example, optical transparency and colorlessness. However, significant processing of wood is required to provide high quality cellulose pulp suitable for cellulose ester end products. This processing typically includes the removal of hemicellulose and lignin. The hemicellulose component of wood is removed by chemical treatment, pre- or post-pulping, and lignin is removed by chemical pulping and bleaching. While the result of such treatment is high quality pulp, the actual pulp yield is low due to the extensive processing. Coupled with the low yield and chemical treatments, as well as capital and operating costs, high quality pulp is expensive.
Accordingly, there exists a need for high quality cellulose esters produced in high yield from low quality cellulose sources, such as high hemicellulose content pulp (e.g., pulp having a hemicellulose greater than about 5 percent by weight based on the total weight of the pulp).
Processes for producing cellulose esters can be categorized as heterogeneous or homogenous.
In heterogeneous processes, wood pulp is treated with an esterifying solution. In such processes, the esterified product becomes dissolved in the solution, which can be readily separated from the insoluble un-esterified pulp, and ultimately recovered from the solution. This type of process can be manipulated to reduce the hemicellulose content and enhance the quality of the product cellulose esters (e.g., by exploiting the preferential esterification of hemicellulose and separating esterified hemicellulose from cellulose that is not (yet) esterified, by conducting the esterification under highly acidic conditions such that hemicellulose content is significantly reduced, as in chemical pulping processes, etc.). However, although these processes can provide suitable cellulose esters, they suffer from reduced yield and additional processing steps.
In homogeneous processes, wood pulp is dissolved in a liquid (e.g., an ionic liquid) and by reaction with an esterifying agent that is also in solution, esterification occurs in solution. Due to all reactants being in solution (rather than the esterifying agent being in solution and the pulp being undissolved and not in solution), homogeneous processes tend to afford greater control and provide more consistent results, as compared with heterogeneous processes. These processes also benefit from starting with high quality pulp (i.e., highly processed pulp containing little to no lignin or hemicellulose). Ionic liquid processes for producing esterified cellulose are described in U.S. Pat. No. 8,067,488.
Therefore, in addition to advantageous (e.g., comparably better handling properties, less color, etc.) cellulose esters produced from high cost, high quality cellulose pulps and the efforts noted above with regard to attempts to produce high quality cellulose esters from lower quality pulp, a need exists for the production of suitable cellulose esters from relatively lower cost, commercially available wood pulp. The present disclosure addresses this need and provides further related advantages.